Let your hair down 放松一下，放开些，尽情享受

【例句】
Let your hair down and enjoy yourself at the party.
在晚会上别拘束，尽情享受一下。

It's wonderful to let your hair down at the weekend after you've been working hard all week.
辛苦工作一周后在周末不拘礼节轻松一下真是太好了。